Why what did he do
the easier question would probably be what didn't he do
Spoilers pls
He’s a bit insane but so lovelyyy
You can be a psychopath and do nothing harmful to others, psychopathy means lack of emotions/empathy and inability to understand emotions the way other people do so you learn how to mimic them. Those people have superficial charm, over the top sense of self-worth, and are a lot of times pathological liars. As seen in Grayson there's also lack of remorse or guilt and a total failure to accept responsibility for any of his actions (kmiyc where he told Chase that he slept with a dog and never revealed the truth, despite seeing what it had done to Chase). Due to not feeling like other people, psychopaths need stimulation most of the time (just take a serial killer as an example). Ofc Grayson doesn't go that far but he's behaviour can be understood through the fact that he's psychopath.
Even tho I don't find the fact that his psychopath here really well explained, yeah he act like one but they are incapable of any kind of bond or love with other human beings, while Grayson will love his fated omega blah blah blah. Author just like over the top toxic shit lol. Other than that, Grayson is just unhinged and don't really understand how unhinged he is, so it hurts everyone around him, he's not evil. But Dane is equally crazy (not clinically tho) so he'll put a leash on him hahaha.
I read the novel, and this MF is a clinical psychopath